A 28-year-old man presents with haematuria, mild hypertension, and normal renal function 2 weeks after a streptococcal pharyngitis. Serum complement C3 levels are low. A 35-year-old woman with a 10-year history of systemic lupus erythematosus (SLE) presents with haematuria, proteinuria, and hypertension. Anti-dsDNA antibodies are positive and serum C3 is also low. Which single feature best distinguishes post-streptococcal glomerulonephritis (PSGN) from lupus nephritis?

Explanation

## Distinguishing PSGN from Lupus Nephritis ### Clinical Course and Prognosis **Key Point:** The most reliable discriminator between PSGN and lupus nephritis is the natural history and reversibility of the disease. PSGN is typically self-limited and resolves spontaneously, whereas lupus nephritis is chronic and progressive without treatment. ### Comparative Table | Feature | PSGN | Lupus Nephritis | |---------|------|----------------| | **Onset** | Acute, post-infectious | Insidious, chronic | | **C3 complement** | Low (transient) | Low (persistent) | | **Natural course** | Self-limited, 6–8 weeks | Progressive, relapsing | | **Renal recovery** | Spontaneous, usually complete | Requires immunosuppression | | **ANA/Anti-dsDNA** | Negative | Positive | | **Prognosis** | Excellent (>95% recovery) | Variable, risk of ESRD | ### Pathophysiology **High-Yield:** PSGN is mediated by **immune complex deposition** (post-infectious) that is **self-clearing** as the infection resolves and antibody production wanes. The complement system is transiently activated but recovers as immune complexes are cleared. Lupus nephritis, by contrast, is driven by **persistent autoimmune dysregulation**. Anti-dsDNA antibodies continuously form immune complexes that deposit in the glomeruli, causing chronic inflammation and progressive glomerular damage. ### Clinical Pearl **Clinical Pearl:** A child or young adult with acute nephritic syndrome 1–3 weeks after streptococcal infection who shows **spontaneous improvement in urinary findings and blood pressure over 6–8 weeks** has PSGN. If renal function remains impaired or proteinuria persists beyond 12 weeks, consider alternative diagnoses (including lupus nephritis). ### Why This Matters for Exam **Tip:** Both conditions present with low C3 and haematuria, making serology the key differentiator. Positive ANA/anti-dsDNA strongly favours lupus. However, the **clinical course** is the gold standard: PSGN resolves spontaneously; lupus nephritis does not without immunosuppression. [cite:Robbins 10e Ch 20]